1. Field of the Invention
The present invention relates generally to an actuation force transmission mechanisms utilizable with a straddle-type vehicle, and more specifically, to an actuation force transmission mechanism operative to transmit actuation force of a shift actuator to a shift shaft of a shift control device for electric control of speed changes.
2. Description of the Related Art
In some electric shift control devices, a conventional foot-operated shift pedal is not used, but a shift actuator (electric motor) is actuated based on a speed change command signal that is output from a shift switch to rotate the shift shaft of a transmission for shift change.
In the case of shift change using a foot-operated shift pedal, repeated shift operations may be required to complete the shift change if a dog in the transmission does not disengage or engage smoothly. However, with an electric shift control device, the shift change might not be made if a dog does not disengage or engage smoothly.
In an attempt to address this problem related to electric shift control devices, a feedback method has been proposed. According to this method, the angle of a shift cam is detected and fed back in order to adjust the operation angle of the shift actuator for ensuring that the dog properly and smoothly disengages and engages. Although beneficial, this method can be problematic due to slow shift speed and the complexity of the device.
This method is also problematic because in order to operate the shift actuator at a predetermined angle in a predetermined period, the shift actuator must keep operating even during abutment of the dog. Due to the abutment with the shift actuator, the dog may tend to rotate with the operation of the shift actuator. Although it is possible to prevent the dog from rotating with the operation of the shift actuator, this requires the interposition of an actuation force transmission mechanism, such as a spring between the shift actuator and the shift shaft. Further, if the load required to disengage the dog cannot be obtained with the spring, the problem arises that the dog cannot be disengaged. In addition, if the stroke amount of the shift actuator needs to be increased, the shift speed is made slower.
In view of the foregoing issues, Japanese Patent Document No. JP-B-3044498 discloses a technique for providing an actuation force transmission mechanism (i.e., a lost motion mechanism) constituted of an elastic member between the shift actuator and the shift shaft. This lost motion mechanism is interposed between a speed reduction gear mechanism (which is positioned between the output shaft and the shift shaft of the shift actuator) and the shift shaft in order to prevent the shift actuator from being overloaded. Thus, instead of being applied to the shift actuator, any overload is applied to the elastic member and results in elastic deformation of the elastic member. Therefore, when the shift shaft is rotationally driven by the resilient force, the shift shaft can be rotationally driven smoothly, without the influence of the inertial mass of the speed reduction gear mechanism. Such a configuration tends to ensure smooth speed change shift operation.
Incidentally, albeit unrelated to electric shift control devices, Japanese Patent Document No. JP-Y-Sho 43-11555 discloses a technique for achieving smooth shift change using a foot-operated shift pedal. This reference teaches a coupling mechanism that is disconnected at a portion between the shift pedal and the shift shaft. Both the disconnected ends of the coupling mechanism are linked via an elastic member and have play equivalent to half the stroke of the shift pedal. With this structure, the dog can be disengaged with operation force of the shift pedal directly applied thereto, and can also be engaged always by the elastic force of the elastic member. This configuration tends to ensure smooth shift change for foot-operated shift pedals.
Despite the beneficial shift control devices described in Japanese Patent Document Nos. JP-B-3044498 and JP-Y-Sho 43-11555, the described actuation force transmission mechanisms, including the elastic member of JP-B-3044498, are both the rotary type and tend to be large in size and restricted in terms of installation location. This restriction on installation location also causes the problem of significant restriction on the layout of the shift actuator. Therefore, there is a need in the art for an actuation force transmission mechanism that allows smooth shift change and is compactly sized in order to mitigate any restriction on installation location and enable easy installation.